This invention pertains to a process of controlling heavies in a catalyst recycle stream. More particularly, this invention pertains to a two-stage process of hydroformylation and product-catalyst separation for controlling heavies in a catalyst recycle stream to the hydroformylation stage.
It is well known in the art that aldehydes can be produced by reacting an olefinically unsaturated compound with carbon monoxide and hydrogen in the presence of a metal-organophosphorus ligand complex catalyst, and that preferred processes involve continuous hydroformylation and recycling of a catalyst solution containing a metal-organophosphorus ligand complex catalyst wherein the metal is selected from Groups 8, 9, or 10. Rhodium is a preferred Group 9 metal. Such art is exemplified by U.S. Pat. Nos. 4,148,830 4,717,775, and 4,769,498. Aldehydes produced by such processes have a wide range of utility, for example, as intermediates for hydrogenation to aliphatic alcohols, for amination to aliphatic amines, for oxidation to aliphatic acids, and for aldol condensation to produce components of plasticizers.
WO 97/07086 discloses a process for recycling a substantially liquid discharge from a hydroformylation. According to this process the liquid and gaseous component-containing hydroformylation discharge is expanded in a flash vessel. The liquid phase from the expansion vessel is fed into the upper part of a column and the gas phase is introduced into the lower part of the column, so that the liquid phase is treated in countercurrent with the gas phase. This process requires a hydroformylation discharge containing butene/butane in sufficient amount. Problems occur if 1-butene rich feeds that lead to high conversions in the hydroformylation are employed.
WO 01/58844 describes a process for working up a liquid output from a continuous hydroformylation, wherein the liquid hydroformylation output is depressurized in a first depressurization stage to a pressure which is from 2 to 20 bar below the reactor pressure, resulting in separation into a liquid phase and a gas phase, and the liquid phase obtained in the first depressurization stage is afterwards depressurized in a second depressurization stage, resulting in separation into a liquid phase comprising essentially high-boiling by-products, the homogeneously dissolved hydroformylation catalyst and small amounts of hydroformylation product and unreacted olefin and a gas phase comprising essentially the major part of the hydroformylation product, unreacted olefin and low-boiling by-products. In this process the difference in pressure between the hydroformylation reactor, first flash and second flash is lower than in processes with a first flash to atmospheric pressure and a further work-up at subatmospheric pressure. Nevertheless, also this process can be further improved with regard to energy consumption.
Commercial hydroformylation of C4 olefins in the presence of a rhodium-triorganophosphine ligand complex catalyst, such as rhodium-triphenylphosphine ligand complex catalyst, is typically conducted in an integrated reaction-separation system similar to that shown in FIG. 1. C4 olefins comprise essentially pure 1-butene or 2-butene streams, as well as mixed C4 raffinate I and raffinate II streams comprising 1-butene, 2-butene, isobutylene, and butane. With reference to FIG. 1, a raffinate stream containing mixed butenes (1) is fed with a stream (2) comprising carbon monoxide and hydrogen (syngas) to a first reactor (Reactor 1). A liquid product stream (3) is removed from the bottom of the first reactor and fed to a second reactor (Reactor 2), while gas stream (4) taken from the top of the first reactor can also be fed into the second reactor (Reactor 2). Each reactor contains a quantity of rhodium-triphenylphosphine ligand complex catalyst and, optionally, free triphenylphosphine ligand. The complex catalyst and optional free ligand are advantageously solubilized in a liquid heavies by-product comprising aldehyde condensation dimers, trimers, and higher oligomers derived from the hydroformylation of the C4 feed. A gas product stream (5) exiting the last reactor can be recycled to the first reactor, or flared, or fed as a fuel to a downstream process. A liquid product stream (6) exiting the last reactor is sent to a vaporizer (also known as a stripper) from which an overhead stream (7) is removed comprising one or more C5 aldehyde product(s), one or more unconverted C4 olefins, unconverted syngas, volatile inerts (e.g., butane), and to some extent heavies by-products. The overhead stream (7) from the vaporizer is condensed at about 40° C. and 10 psig (69 kPa), and the resulting liquid stream (8) is sent to a refining zone (unit not shown) for C5 separation and purification. A vent stream (9) removes volatiles from the condenser. These volatiles comprise mostly nitrogen, carbon monoxide, hydrogen, and less than 1 percent aldehyde products. The vent gases can be flared, routed to a vent recovery stream, or routed to a downstream plant fuel stream. A catalyst recycle stream (10) containing the rhodium-triphenylphosphine ligand complex catalyst and, optional, free triphenylphosphine ligand dissolved in a liquid heavies by-product is obtained from the vaporizer as a liquid tail stream and recycled usually to the first hydroformylation reactor (Reactor 1). The vaporizer operating conditions are adjusted so that the production rate of heavies in the reaction system essentially equals their removal rate in the vaporizer. The vaporizer is operated at about 135° C. and super-atmospheric pressure. Under these vaporizer conditions, the rhodium-triphenylphosphine ligand complex catalyst is thermally stable. Moreover, the heavies concentration in the catalyst recycle stream to the first reactor usually remains constant, avoiding a build-up of heavies by-products in the recycle stream to the hydroformylation reactor(s).
Present day hydroformylation processes prefer to replace the triorganophosphine ligand with an organophosphite ligand, because the latter possesses higher activity and produces a higher ratio of normal to branched isomeric aldehyde products. The prior art describes various mono, bis-, and poly-organophosphite ligands for use in modern-day hydroformylation processes. Disadvantageously, organophosphite ligands tend to be less stable as compared with triorganophosphine ligands, that is, more sensitive to thermal degradation. Rhodium-organophosphite catalysts, for example, tend to degrade thermally in the vaporizer at operating conditions suitable for the rhodium-triphenylphosphine ligand. Consequently, it is desirable to operate the vaporizer at a temperature lower than 135° C. in order to minimize thermal degradation of the organophosphite ligand.
Operating the vaporizer at a temperature lower than 135° C. requires the use of sub-atmospheric pressures in order to remove the heavies overhead to the desired extent. The quantity of heavies in the tail stream from the vaporizer should be sufficient to solubilize the catalyst and optional free ligand for recycle in a liquid stream back to the hydroformylation reactors; however, a build-up of heavies in the recycle stream is desirably avoided. Thus, the heavies desirably are removed overhead from the vaporizer at essentially the same rate at which they are formed in the hydroformylation stage, in order to avoid increasing quantities of heavies being returned to the hydroformylation reactors where the heavies would occupy ever increasing reactor volume and reduce productivity. Thus, if the organophosphite catalyst is to be stabilized, and heavies are to be removed to the extent desirable, the vaporizer is required to operate at a temperature lower than 135° C. and at sub-atmospheric pressure. Disadvantageously, condensation of the overhead stream taken from the vaporizer becomes problematic at sub-atmospheric pressure. Condensation temperatures of 0° C. or lower require a costly refrigeration unit and add complexity to the overall system. It would be desirable to avoid this expense and complexity by using a simple water cooling condensation unit for condensing the overhead stream from the vaporizer; but it is not apparent from the prior art how to employ conventional water cooling when desirable organophosphite ligands are employed in the hydroformylation stage.
Moreover, the use of sub-atmospheric pressure requires expensive equipment, such as compressors or turbines, having a high energy consumption. It would therefore be desirable to avoid such process steps with high energy consumption and/or that afford a high expenditure on equipment.
Working at sub-atmospheric pressure bears a certain risk of air leakage into the apparatus which might cause deterioration of the catalyst activity and/or increase of catalyst decomposition. Hence, it would be desirable to prevent air leakage into the apparatus.